Pb-FREE SOLDER-CONNECTED STRUCTURE

ABSTRACT

Provided are a bonded structure by a lead-free solder and an electronic article comprising the bonded structure. The bonded structure has a stable bonding interface with respect to a change in process of time, an enough strength and resistance to occurrence of whiskers while keeping good wettability of the solder. In the bonded structure, a lead-free Sn—Ag—Bi alloy solder is applied to an electrode through an Sn—Bi alloy layer. The Sn—Bi alloy, preferably, comprises 1 to 20 wt % Bi in order to obtain good wettability of the solder. In order to obtain desirable bonding characteristics having higher reliability in the invention, a copper layer is provided under the Sn—Bi alloy layer thereby obtaining an enough bonding strength.

This application is a Divisional application of prior application Ser.No. 12,773,128, filed May 4, 2010, which is a Continuation applicationof application Ser. No. 11/331,220, filed Jan. 13, 2006, which is aDivisional application of application Ser. No. 09/971,566, filed Oct. 9,2001, which is a Divisional application of application Ser. No.09/581,631, filed Jun. 15, 2000, now abandoned, which is a NationalStage application under 35 USC 371 of International (PCT) ApplicationNo. PCT/JP98/05565, filed Dec. 9, 1998. The contents of Ser. No.09/581,631 are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a bonded structure by a lead-freesolder, in which an electronic device is bonded to an electrode of alead frame, etc. by means of the lead-free solder of low toxicity, andan electronic article with the bonded structure.

BACKGROUND ART

In order to produce an electric circuit board by bonding electricdevices (e.g. LSIs) to a circuit board made of an organic material, forexample, conventionally, there has been used a eutectic Sn—Pb alloysolder, another Sn—Pb alloy solder which has a chemical composition anda melting point each close to that of the eutectic Sn—Pb alloy solder,and other solder alloys which are obtained by adding small amounts ofbithmuth (Bi) and/or silver (Ag) to the solders recited above. Thesesolders comprise about 40 wt % Pb and have a melting point of about 183°C., which permit soldering at 220-240° C.

With regard to electrodes of electronic devices, such as QFP (Quad FlatPackage)-LSIs, to be soldered, there have been usually used those madeof 42 alloy which is an Fe—Ni alloy and on which a layer of 90 wt %Sn-10 wt % Pb alloy (hereinafter referred to “Sn-10Pb”) is formed. Thisis because such electrodes have good wettability, good preservation andno problem of formation of whiskers.

However, the lead (Pb) in the Sn—Pb solders is a heavy metal harmful tohumans and pollution of the global environment caused by dumping oflead-containing products and their bad effect on living things havepresented problems. The pollution of the global environment byelectrical appliances occurs when lead is dissolved by rain, etc. fromthe dumped lead-containing electrical appliances exposed to sunlight andrain. The dissolution of Pb tends to be accelerated by the recent acidrain. In order to reduce environmental pollution, therefore, it isnecessary to use a lead-free soldering material of low toxicity notcontaining lead as a substitute for the above eutectic Sn—Pb alloysolder which is used in large quantity and to employ a structure of theelectrode of a device not containing lead as a substitute material toreplace the Sn-10Pb layer provided on the electrode of a device. AnSn—Ag—Bi alloy solder is a promising candidate as a lead-free solderingmaterial in terms of low toxicity, obtainability for raw materials,production cost, wettability, mechanical properties, reliability, etc.Soldering is usually performed at a temperature of about 220-240° C. soas to produce compounds between an electrode of a component and asolder, and between an electrode of a board and a solder. From this,because the bonding interfaces differs from one another depending upondifferent kinds of combinations of solder materials and electrodematerials of components, an electrode material suitable to therespective solder is required in order to obtain a stable bondinginterface.

An object of the present invention is to provide a bonded structure by alead-free-solder, in which a lead free Sn—Ag—Bi alloy solder having lowtoxicity is used for electrodes of lead frames, etc. and which has astable bonding interface and an enough bonding strength.

Another object of the invention is to provide an electronic article withutilization of a lead-free Sn—Ag—Bi alloy solder having low toxicity,which has a stable bonding interface with respect to a change in processof time and a strength high enough to withstand stress generated inbonded portions by soldering due to a difference in thermal expansioncoefficient between electric devices and a board, a work of dividing theboard after soldering, warping of the board during the probing test,handling and so on.

A further object of the invention is to provide a bonded structure andan electronic article with utilization of a lead-free Sn—Ag—Bi alloysolder having low toxicity, which has an enough bonding strength whileensuring resistance to formation of whiskers, wettability of the solderand so on.

DISCLOSURE OF INVENTION

Thus, in order to achieve the objects of the invention, there isprovided a bonded structure by a lead-free solder of an Sn—Ag—Bi alloywhich is applied to an electrode through an Sn—Bi alloy layer.

In the invention bonded structure with utilization of the lead-freesolder, the Sn—Bi alloy layer comprises 1 to 20 wt % Bi.

The invention bonded structure comprises a copper layer between theelectrode and the Sn—Bi alloy layer.

In the invention bonded structure, the electrode is made of coppermaterial.

In the invention bonded structure, the electrode is of a lead made of anFe—Ni alloy or a copper alloy.

In the invention bonded structure, the lead-free Sn—Ag—Bi alloy soldercomprises Sn as a primary component, 5 to 25 wt % Bi, 1.5 to 3 wt % Agand up to 1 wt % Cu.

The invention is also directed to an electronic article which comprisesa first electrode formed on an electronic device and a second electrodeformed on a circuit board, the both types of electrodes being bondedwith each other by a solder, wherein an Sn—Bi alloy layer is formed onthe first electrode and the solder is made of a lead-free Sn—Ag—Bialloy.

In the invention electronic article, the Sn—Bi alloy layer comprises 1to 20 wt % Bi.

The invention electronic article comprises a copper layer between thefirst electrode and the Sn—Bi alloy layer.

In the invention electronic article, the first electrode is made ofcopper material.

In the invention electronic article, the electrode is of a lead made ofan Fe—Ni alloy or a copper alloy.

In the invention electronic article, the lead-free Sn—Ag—Bi alloy soldercomprises Sn as a primary component, 5 to 25 wt % Bi, 1.5 to 3 wt % Agand up to 1 wt % Cu.

The invention is also directed to a bonded structure by a lead-freesolder, which comprises an electrode, wherein the lead-free solder is ofan Sn—Ag—Bi alloy comprising Sn as a primary component, 5 to 25 wt % Bi,1.5 to 3 wt % Ag and up to 1 wt % Cu, which is applied to the electrode.

As can be understood from the above, according to the present invention,it is possible to ensure a stable bonding interface having an enoughbonding strength by applying the lead-free Sn—Ag—Bi alloy solder of lowtoxicity to an electrode such as a lead frame. With utilization of thelead-free Sn—Ag—Bi alloy solder of low toxicity, it is also possible toensure a bonding interface which is stable with respect to a change inprocess of time and which has a high enough strength to withstand stressgenerated in bonded portions by soldering due to a difference in thermalexpansion coefficient between electric devices and a board, a work ofdividing the board after soldering, warping of the board during theprobing test, handling and so on. Further, with utilization of thelead-free Sn—Ag—Bi alloy solder of low toxicity, it is possible toensure a bonding interface which has an enough strength and goodresistance to occurrence of whiskers by forming sufficient fillets whilekeeping good wettability at a soldering temperature of, for example,220-240° C.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional view of a lead for a QFP-LSI according tothe invention;

FIG. 2 shows a cross-sectional view of a lead for a TSOP according tothe invention;

FIG. 3 schematically shows a testing way of evaluating solder-bondingstrength;

FIG. 4 shows evaluation results of fillet strength with regard tovarious types of metallized leads according to the invention;

FIG. 5 shows evaluation results of wetting time with regard to varioustypes of metallized leads according to the invention;

FIG. 6 shows evaluation results of wetting force with regard to varioustypes of metallized leads according to the invention;

FIG. 7 shows evaluation results of fillet strength in the case wherethere is formed a copper layer according to the invention;

FIG. 8 shows evaluation results of flat portion strength in the casewhere there is formed a copper layer according to the invention;

FIG. 9 shows an observation result of an interface region of a solderand a lead of an Fe—Ni alloy (i.e. 42 alloy) on which an Sn-10Pb alloyplating is provided according to the prior art, wherein (a) is across-sectional view of the interface region, and (b) are fracturedsurfaces at the lead side and the solder side, respectively;

FIG. 10 shows an observation result of an interface region of a solderand a lead of an Fe—Ni alloy (i.e. 42 alloy) on which an Sn-4Bi alloyplating is provided according to the invention, wherein (a) is across-sectional view of the interface region, and (b) are fracturedsurfaces at the lead side and the solder side, respectively; and

FIG. 11 shows an observation result of an interface region of a solderand a lead of an Fe—Ni alloy (i.e. 42 alloy) of the invention on whichan under copper layer and an upper Sn-4Bi alloy plating is providedaccording to the invention, wherein (a) is a cross-sectional view of theinterface region, and (b) are fractured surfaces at the lead side andthe solder side, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a description of embodiments according to the inventionwill be provided.

One embodiment of the invention is an electronic article, comprising afirst and a second electrodes both of which are bonded with each otherby means of a lead-free solder having low toxicity, the first electrodebeing a QFP lead, a TSOP lead or the like in an electronic device suchas a semiconductor device (e.g. LSI), for example, and the secondelectrode being on a circuit board.

Another embodiment of the invention is a bonded structure comprising afirst and a second electrodes both of which are bonded with each otherby means of a lead-free solder having low toxicity.

The lead-free solder having low toxicity can be of an Sn—Ag—Bi alloy.With utilization of the Sn—Ag—Bi alloy, it is required to obtain abonding interface which is stable with respect to a change in process oftime and has a bonding strength high enough to withstand stressgenerated in solder-bonded portions due to a difference in thermalexpansion coefficient between an electronic device and a circuit board,a work of dividing the board after soldering, warping of the boardduring the probing test, handling and so on. It is also required toobtain an enough bonding strength with utilization of the lead-freeSn—Ag—Bi alloy solder by forming a sufficient fillet shape whileensuring enough wettability at 220-240° C., which is a suitablesoldering temperature with respect to heat resistance of circuit boardsand electronic devices. If the solder has inferior wettability, asufficient fillet shape can not be obtained resulting in that an enoughbonding strength is not obtained or a more active flux is requiredleading to an adverse influence on insulation resistance. Furthermore,it is also necessary to ensure resistance to formation of whiskers, etc.because short-circuit occurs between electrodes if whiskers aregenerated and grow on the electrode surface treated by plating, etc.

As shown in FIGS. 1 and 2, an Sn—Bi layer 2 is formed on the surface ofan electrode 1 of a lead to obtain enough bonding strength as theelectrode structure of the invention. Next, a selection of an electrodestructure of the invention will be described. Such selection was made byevaluating mainly bonding strength, wettability and resistance tooccurrence whiskers based on the above requirements.

First, the result of an examination of the bonding strength obtainedbetween an Sn—Ag—Bi alloy solder and various kinds of electrodematerials are described. An outline of the experiment is illustrated inFIG. 3. Sample leads 4 were formed by plating lead-free materials of Sn,Sn—Bi, Sn—Zn and Sn—Ag alloys, respective which are considered to beusable as alternative materials for the the conventional Sn-10 Pb alloylayer, onto leads each of which is an electrode made of an Fe—Ni alloy(42 alloy). Besides, an evaluation was also performed for combinationswith the conventional Sn-10 Pb alloy plating. The respective examplelead 4 was 3 mm wide and 38 mm long. It was bent to form right angles sothat the length of the soldering section is 22 mm. The plating thicknesswas approximately 10 μm for each composition. The respective examplelead 4 was soldered to a Cu pad (Cu electrode) 7 on a glass epoxysubstrate 6, which is a circuit board, with utilization of a lead-freesolder 5 of a 82.2 wt % Sn-2.8 wt % Ag—15 wt % Bi alloy (hereinafterreferred to as Sn-2.8Ag-15Bi).

The Cu pad (Cu electrode) 7 on the glass epoxy substrate 6 had a size of3.5 mm×25 mm. The solder 5 was provided in the form of a foil of 0.1mm×25 mm×3.5 mm. More specifically, the solder foil 5 was placed on theCu pad 7 on the glass epoxy substrate 6 and the example lead 4 beingbent with the right angle was placed on the solder foil 5. Soldering wasperformed in the air at a maximum temperature of 220° C. afterpreheating at 140° C. for 60 seconds. A rosin flux containing chlorinewas used when soldering. After soldering, cleaning was conducted with anorganic solvent. The pull test was conducted in three cases; i.e., asample lead immediately after soldering, another example lead exposed toa high temperature of 125° C. for 168 hours after soldering takingaccount of the deterioration of bonding strength due to a change withthe passage of time, and a further sample lead after soldering followingthe exposure thereof to 150° C. for 168 hours to investigate bondingstrength in the case where wettability of lead is deteriorated. In thepull test, the example lead was pulled vertically at a rate of 5mm/minute by gripping its distal end while the substrate is fixed. Thena maximum strength and a generally saturated constant strength weredetected as a fillet strength and a flat portion strength, respectively,for the example lead of each composition. The test was conducted tentimes for each condition to determine an average value.

The test results of the fillet strength of the example lead of eachcomposition are shown in FIG. 4. In plastic package devices such asordinary QFP-LSIs, it is necessary that fillet strength be at leastapproximately 5 kgf in consideration of a difference in thermalexpansion coefficient of printed-circuit board. From this, it becameapparent that an adequate bonding interface cannot be obtained in thecase of Sn—Zn, Sn—Ag and Sn—Pb alloy layers although fillet strength ofnot less than 5 kgf was obtained with the example leads in which an Snlayer or Sn—Bi layers other than Sn-23Bi layers containing 23 wt % Biare plated on the Fe—Ni alloy (42 alloy). In addition to these exampleleads, further three types of example leads were prepared by providingan Ni plating layer having a thickness of about 2 μm onto the 42 alloyand plating the Ni layer with Au layer, a Pd layer, and a Pd layer witha further Au layer, respectively. Soldering was performed in the samemanner and bonding strength was investigated. However, enough filletstrength was incapable of being obtained as shown in FIG. 4.Accordingly, it became apparent that it is necessary to apply an Sn—Bilayer to a lead of an electrode.

Wettability to the Sn-2.8Ag-15Bi solder was tested by the meniscographmethod in the Sn—Bi alloy plated leads which showed enough bondingstrength in the above pull test conducted on example leads of variouscompositions. A flux of less activity was used in order to investigatewettability. Test pieces were obtained by cutting the above exampleleads into a length of 1 cm. The wettability test was conducted underthe test conditions: a solder bath temperature of 220° C., an immersionspeed of 1 mm/minute, an immersion depth of 2 mm and an immersion timeof 20 seconds. The time which elapses till the load recovers to 0 (zero)was regarded as wetting time and the load after immersion for 20 secondswas regarded as wetting force. Wettability was determined in two cases:a lead immediately after plating and a lead exposed to 150° for 168hours after plating. Measurements were made ten times for each testcondition to obtain an average value.

The wetting time and wetting force for each composition are shown inFIG. 5 and FIG. 6, respectively. It became apparent from the result ofwetting time shown in FIG. 5 that the higher the Bi content, the betterwettability in the Sn—Bi alloy plated leads tested immediately afterplating, while wettability is deteriorated at below 1 wt % Bi and at 23wt % Bi when the leads are exposed to a high temperature of 150° for 168hours. It can be said that at Bi contents of below 1 wt %, wettabilitywas low because the wetting time became long while the wetting force wasensured as shown in FIG. 6. Therefore, it became apparent that adesirable Bi content is from 1 to 20 wt % in order to obtain sufficientwettability even with the Sn—Bi alloy layer.

Stress generated in the interface is high when materials with a greatdifference in thermal expansion coefficient are bonded together, whenmaterials are used in an environment of great temperature difference,and the like. The bonding strength in the interface must beapproximately 10 kgf or more in order to ensure sufficient reliability.Therefore, it became evident from FIG. 4 that fillet strength of 10 kgfor more cannot be obtained by directly providing an Sn—Bi layer onto theFe—Ni alloy (42 alloy). It is believed that this is because thecompounds at the interface are not sufficiently formed. Therefore, a Cuplating layer of about 7 μm on average was applied to the Fe—Ni alloy(42 alloy) and an Sn—Bi alloy plating layer was applied to this Cu layerin order to raise the reactivity with the solder in the interface andbonding strength was measured. The fillet strength, in the case of no Culayer, is also shown in FIG. 7. Bonding strength of not less than 10 kgfwas obtained with the exception of the case of 23 wt % Bi and the effectof the underlayer of Cu was capable of being verified. By adopting thiselectrode structure it was possible to obtain a bonding strength ofabout 12.1 kgf or more that is obtained immediately after soldering of alead made of the 42 alloy on which an Sn-10Pb alloy layer is formed,which is soldered by means of a eutectic Sn—Pb alloy solder, and whosebonding strength is also shown as a comparative solder in FIG. 7.Furthermore, as shown in FIG. 8, flat portion strength was also capableof being improved by forming a Cu layer under the Sn—Bi alloy layer. TheCu layer may be applied to the 42 alloy as described above when a leadframe of 42 alloy is used. However, when a Cu lead frame is used, thislead frame may be allowed to serve as the Cu layer or a further Cu layermay be formed in order to eliminate the effect of other elements whichmay sometimes be added to the lead frame material to improve rigidity.The wettability of the example leads to which this Cu layer is appliedis also shown in FIGS. 5 and 6. There is scarcely any effect of the Culayer and sufficient wettability was capable of being obtained at 1-20wt % Bi, although wettability also deteriorated at Bi contents of notmore than 1 wt % when the lead frames were exposed to a hightemperature. Incidentally, an Sn-2.8Ag-15Bi was used in the examplesshown in FIGS. 7 and 8. However, the formation of an underlayer of Cu iseffective in improving bonding strength even in systems of low Bicontent, for example, an Sn-2Ag—7.5Bi-0.5Cu alloy.

The method of application of the above Sn—Bi alloy and Cu layers is notlimited to plating and these layers can also be formed by dipping,deposition by evaporation, roller coating or metal powder application.

Thus, in order to investigate the reason why various types of theelectrode materials have different strengths from one another,cross-sectional surfaces of bonding portions were observed afterpolishing. Further the fractured surfaces of samples subjected to thepull test were observed under an SEM. The results obtained in thetypical combinations are described below.

First, FIG. 9 shows an observation result in the case where a leadobtained by applying an Sn-10Pb alloy plating layer directly onto theconventional Fe—Ni alloy (42 alloy) is bonded using an Sn—Ag—Bi alloysolder. In this combination, Pb—Bi compounds agglomerated at theinterface and fracture occurred in the interface between the 42 alloyand the solder. A small amount of Sn was detected on the fractured 42alloy surface of the lead and it is believed that the Sn in the solderformed compounds with the 42 alloy of lead. It is believed, therefore,that agglomaration of the above compounds of Pb and Bi at the interfacereduced the contact area between Sn and 42 alloy, greatly weakeningbonding strength.

Next, FIG. 10 shows an observation result in the case where the Sn-10Pballoy plating layer was replaced with an Sn-4Bi alloy plating layer. Thecompound layer foamed in the interface was thin and fracture occurredsimilarly at the interface between 42 alloy and solder. However, Biremained granular crystals, which do not cause a decrease in the area ofbond between Sn and 42 alloy so much as in the case of an Sn-10Pb. It isbelieved that this is the reason why bonding strength of not less than 5kgf was capable of being obtained. Auger analysis revealed that the thencompound layer is an Sn—Fe layer of about 70 nm.

FIG. 11 shows an observation result in the case where a Cu layer wasformed on under the Sn-4Bi layer. It was found that a thick layer ofcompounds of Cu and Sn is formed in the interface. Fracture occurred inthe interface between this compound layer and the solder or in thecompound layer. The fractured surface was almost flat in the case shownin FIG. 10 where the Sn—Bi alloy layer was directly formed on the 42alloy lead, whereas it was uneven in the case where the Cu layer waspresent. For this reason, it is believed that this difference in thefractured surface resulted in the improvement in bonding strength.Incidentally, similar investigation results were obtained also fromother Sn—Ag—Bi alloy solder compositions.

Occurrence of whiskers was investigated for the above example leads ofeach composition. The formation of whiskers was observed on the surfacesof the example leads to which an Sn—Zn alloy plating layer was applied.It has been hitherto said that Sn plating presents a problem inresistance to the formation of whiskers. However, the occurrence ofwhiskers was not observed in the Sn—Bi alloy layers and there was noproblem in resistance to formation of whiskers.

Accordingly, with the use of the electrode structures of the invention,the bonding portions excellent in bonding strength, wettability andresistance to occurrence of whiskers can be obtained by means ofSn—Ag—Bi alloy solders.

The reason why Sn—Ag—Bi solders containing Sn as a primary component, 5to 25 wt % Bi, 1.5 to 3 wt % Ag and optionally 0 to 1 wt % Cu wereselected is that solders of the composition in these ranges permitsoldering at 220-240° C. and that these solders have almost the samewettability as eutectic Sn—Ag alloy solders, which have hitherto beenfield proven for Cu, and provide sufficient reliability at hightemperatures. More specifically, Sn—Ag—Bi alloy solders have acomposition (a ternary eutectic alloy) which melt at approximately 138°C. when the Bi content is not less than approximately 10 wt % and it isconcerned about that these portions might have an adverse influence onreliability at high temperature. However, the precipitation of a ternaryeutectic composition is controlled to levels that pose no problem inpractical use and high-temperature strength at 125° C. is also ensured.Accordingly, practical and highly reliable electronic articles can beobtained by soldering the above electrode using the solder of thiscomposition.

Example 1

The cross-sectional structure of a lead for QFP-LSI is shown in FIG. 1.This illustrates a part of the cross-sectional structure of the lead. AnSn—Bi alloy layer 2 was formed on a lead 1 which is of an Fe—Ni alloy(42 alloy). The Sn—Bi alloy layer 2 was formed by plating and itsthickness was about 10 μm.

The Bi content of Sn—Bi alloy plating layer was 8 wt %. The aboveQFP-LSI having this electrode structure was soldered to a glass epoxysubstrate, which is a circuit board, with utilization of anSn-2.8Ag-15Bi-0.5Cu alloy solder. Soldering was carried out in a reflowfurnace of a nitrogen environment at a peak temperature of 220° C. Itwas possible to obtain bonding portions having sufficient bondingstrength. Similarly, a reflow soldering was carried out on a glass epoxysubstrate in the air at 240° C. with utilization of anSn-2Ag-7.5Bi-0.5Cu alloy solder. Bonded portions produced by reflowheating have high reliability especially at a high temperature.

Example 2

The cross-sectional structure of a TSOP lead is shown in FIG. 2 which isa part of the lead structure. A Cu layer 3 is formed on a lead 1 whichis of an Fe—Ni alloy (42 alloy) and an Sn—Bi alloy layer 2 is formed onthis Cu layer. The Sn—Bi alloy layer 3 and Sn—Bi layer 2 were formed byplating. The thickness of the Cu layer 3 was about 8 μm and that of theSn—Bi plating layer was about 10 μm. The Bi content of Sn—Bi alloyplating layer was 5 wt %. Because of high rigidity of the TSOP lead,when it is used at a high temperature or under a condition that heatgeneration occurs in the device itself, stress generated at theinterface is greater as compared with the QFP-LSI. In such cases, it isnecessary to form an interface with sufficient bonding strength highenough to withstand this interface stress and the Cu layer under theSn—Bi layer is effective for this purpose.

The TSOP was soldered to a printed-circuit board in a vapor reflowfurnace with utilization of an Sn—Ag—Bi alloy solder and the thermalcycle test was conducted. The test was conducted under the two testconditions: one hour per cycle of −55° C. for 30 minutes and 125° C. for30 minutes, and one hour per cycle of 0° C. for 30 minutes and 90° C.for 30 minutes. After 500 cycles and 1,000 cycles the cross section wasobserved and the condition of formation of cracks was investigated. Thecycle test result of crack occurrence was compared with a case where aTSOP of the same size having 42 alloy on which an Sn-10Pb alloy layer isdirectly formed, was soldered using a eutectic Sn—Pb alloy solder.Although cracks were formed early in the thermal cycles of −55° C./125°C., no problems arose with the thermal cycles of 0° C./90° C. and abonding interface which is adequate for practical use was obtained.

Example 3

The electrode structures according to this invention can also be appliedin an electrode on a board. For example, solder coating is effective inimproving the solderability of boards. Conventionally, there have beenused lead-containing solders such as a eutectic Sn—Pb alloy solder.Thus, the Sn—Bi alloy layer according to the invention can be used tomake the solder for coating lead-free. Furthermore, because theelectrode of a board is made of copper, sufficient bonding strength canbe obtained when an Sn—Ag—Bi alloy solder is used. An example in whichthis structure is applied is shown; an Sn-8Bi alloy layer of about 5 μmwas formed by roller coating on a Cu pad (Cu electrode) on a glass epoxysubstrate, which is a circuit board,

Wettability to boards and bonding strength were improved, because thesolder layer was formed.

INDUSTRIAL APPLICABILITY

An electrode structure can be realized, which is suitable for anSn—Ag—Bi alloy solder excellent as a lead-free material.

A bonded structure by a lead-free solder can be realized withutilization of a lead-free Sn—Ag—Bi alloy solder, in which an bondinginterface which is stable and has sufficient bonding strength can beobtained.

An electronic article can be realized with utilization of a lead-freeSn—Ag—Bi alloy solder of low toxicity, which has a bonded structure bythe lead-free solder, which can provide a stable bonding interface withrespect to a change in process of time and a strength high enough towithstand stress generated in bonded portions by soldering due to adifference in thermal expansion coefficient between electric devices anda board, a work of dividing the board after soldering, warping of theboard during the probing test, handling and so on.

With utilization of a lead-free Sn—Ag—Bi alloy solder of low toxicity,it is possible to obtain sufficient bonding strength by forming adequatefillets while ensuring sufficient wettability, for example, at 220-240°and to ensure resistance to formation of whiskers, etc.

Soldering electronic devices with utilization of an Sn—Ag—Bi soldermakes it possible to obtain an interface which has sufficient bondingstrength and to ensure wettability which is sufficient for practicaluse. There is no problem in resistance to formation of whiskers. Thus itis possible to realize lead-free electrical appliances which areenvironmentally friendly by using the same equipment and process asconventionally.

What is claimed is:
 1. A structure to be mounted on a substrate via aPb-free solder, comprising: an LSI; a lead; and a Bi-containing layerformed on the lead and constituted by an alloy consisting essentially ofSn and Bi, wherein a contained amount of Bi in the alloy is greeter thanor equal to 1 wt %, and less than or equal to 5 wt %; and wherein theBi-containing layer is a layer separate from the Pb-free solder.
 2. Thestructure according to claim 1, wherein the lead is comprised of a Cualloy.
 3. The structure according to claim 1, wherein the lead iscomprised of a Fe—Ni alloy.
 4. The structure according to claim 3,wherein a Cu layer is formed between the lead and the Bi-containinglayer.
 5. The structure according to claim 1, wherein the structure isconstituted by a Thin Small Outline Package (TSOP).
 6. The structureaccording to claim 2, wherein the structure is constituted by a QuadFlat Package (QFP).
 7. The structure according to claim 1, wherein theBi-containing layer is formed directly on the lead.
 8. The structureaccording to claim 1, wherein the lead contains Cu.
 9. The structureaccording to claim 1, wherein the Bi-containing layer is a layer inaddition to the Pb-free solder.
 10. The structure according to claim 1,which additionally includes a Pb-free solder, the Bi-containing layerbeing provided between the lead and the Pb-free solder.
 11. Thestructure according to claim 1, wherein the Bi-containing layer is alayer that improves the wettability of the Pb-free solder to the lead.12. The structure according to claim 1, wherein the substrate is a glassepoxy substrate.